The present invention relates to a method and a device for producing snow from flows of humid air and cold air.
Traditional snow cannons, which are widely used in ski resorts, do not produce snow as such, but only specific types of snow, mainly corresponding to fully or partly frozen water droplets (being called “sleet” or “graupel” when occurring in nature). In natural clouds, snowflakes gradually grow in the course of the re-sublimation (i.e., the phase transition from vapor phase to solid phase), and only nuclei required for the initiation of ice crystal growth can be formed by freezing. In traditional snow cannons, water is fed into a nozzle together with pressurized air, whereby the water is atomized into very fine droplets and is released into the surrounding air, where, if the ambient temperature is sufficiently low, the water will freeze to form ice, which will then fall to the ground (see, for example, F. Hahn “Künstliche Beschneiung im Alpenraum” (“The production of artificial snow in the Alpine region”), Cipra International, 2004; and M. Meier, “Produktion von naturidenti-schem Schnee” (“Producing nature-identical snow”), Diploma Thesis, ETH Zurich, 2006). An improved and modern embodiment of such a snow cannon is disclosed by L. Nilsson in European Patent Application Publication EP 1,710,519 A1, for example, while European Patent Application Publication EP 1,065,456 A1 describes a snow cannon which is operated within a closed space, such as in a tent, in order to make it possible to influence the characteristics of the flow of cold air. U.S. Pat. No. 3,257,815 describes a device for producing snow from atomized water and cold air, wherein the water mist which is falling down is frozen by contacting cold air rising from the bottom; the artificial snow is then released at the bottom end of the device.
However, artificial snow produced in this way, as well as ski slopes covered with this kind of artificial snow, have several drawbacks. First, the amounts of energy required for the production of this kind of artificial snow and the emission of noise during the production procedure are enormous. Second, the ice crystals so formed are simply not snowflakes but frozen ice droplets. This results in an increased risk of injury for skiers and snowboarders, if they fall over on such icy slopes, as well as an impairment of the skiing/snowboarding experience, as most skiers and snowboarders prefer freshly fallen snow, i.e., slopes with a cover of loose, soft, “fluffy” snow having a low density, which is also referred to as “dendritic snow.” Additionally, such artificial divergence of the snow cover properties from the natural ones, formed from clouds, often implies an additional pressure on the environment, which, however, is not yet fully understood as there are very few environmental studies on this topic (see, for example, C. Rixen, V. Stoeckli, W. Ammann, “Does artificial snow production affect soil and vegetation of ski pistes? A Review,” Perspectives in Plant Ecology, Evolution and Systematics, 5(4): 219-230 (2003)).
In nature, such dendritic snowflakes are formed while they are floating in the high air layers of the atmosphere and probably while they fall down from great altitudes, when ice crystals formed from air being supersaturated with water vapor are slowly growing into dendrites or similar snow crystals (see C. Fierz, R. L. Armstrong, Y. Durand, P. Etchevers, E. Greene, D. M. McClung, K. Nishimura, P. K. Satyawali, S. A. Sokratov, “The International Classification for Seasonal Snow on the Ground,” IHP-VII Technical Documents in Hydrology, No. 83, IACS Contribution No. 1, UNESCO-IHP, Paris, 2009). This growing process, however, takes several minutes, which is why, so far, approaches for producing artificial or even nature-identical snow did not succeed in producing any dendritic snow at all or only yielded a few grams thereof.
Japanese patent application publication JP 46-7151 A also describes a method for producing snow using water mist and cold air. In this case, sleet and ice crystals are produced in the first of two freezing chambers which are kept at a temperature of −35 to −30° C. The sleet mixture is kept floating for several minutes in both chambers by a flow of cold air, which is blown in from the bottom, and is converted into “snow” in the second chamber. The thus obtained snow is moved to a third chamber, which is kept at a temperature of −15 to −20° C. by blowing down cold air onto the snow from above, in order to prevent the snow from being reconverted into ice, where it is stored.
In this case, the sleet, i.e. ice grains, obtained by this method may agglomerate to form larger crystal structures in the course of the “conversion phase” which lasts several minutes. However, the formation of the above described “fluffy” or dendritic snow is not possible in this case either, because this type of snow is only produced from an atmosphere oversaturated with vapor. It is not possible to create such an atmosphere by the method according to JP 46-7151 A in the first place: because of the low temperature (−35 to −30° C.) in the first freezing chamber, the water droplets spontaneously freeze into ice droplets and sleet. Subsequently, the humidity in each chamber will correspond to the respective temperature therein, but it will not be possible to achieve an oversaturation of the atmosphere, especially because the atmosphere is not further cooled in the subsequent chambers, but also additional non-humidified “dry” air is supplied and, in the third chamber, significantly warmer air is used.
Russian published patent application 1,617,272 A1 describes a “snow generator,” in which a flow of cold air is divided, so that one of the partial air flows passes through a humidifier and is afterwards additionally contacted with water mist while moving upwards into a snow production chamber. This partial air flow, which contains both vapor and water droplets and which has been warmed due to the contact with water, is mixed with the second partial flow of cold air, which creates an atmosphere oversaturated with vapor, and first snow starts to form. Then, the combined air flow passes through the production chamber where it is further cooled by cooling tubes in order to form more snow. After that, the snow is blown out into a cyclone, where it is deposited and thus separated from the air flow, which is recycled to provide the flow of cold air. Concerning the residence time of the snow thus obtained in the production chamber or concerning its structure, no information is given.
For this reason, it may be assumed that the water droplets present in the partial flow of humidified air again become sleet, when the flow of humidified air is mixed with the flow of cold air, the sleet increasing in size when passing through the production chamber, due to water molecules precipitating from the oversaturated atmosphere. As there are no measures for increasing the residence time in the production chamber, it will again not be possible to produce “fluffy,” dendritic snow using this device.
M. Meier, supra, describes an approach for producing nature-identical snow using a “snow machine” in which cold air is blown over a heated water basin within a cooled closed space, which causes the air to take up moisture. The air is then cooled while it rises and thus becomes supersaturated with water. In the top area of the machine, the water condenses on nylon threads, which leads to the growth of snow crystals thereon. As soon as these crystals have reached a certain size, they fall down from the threads and are collected in a drawer positioned underneath. The snow-flakes may have a more or less dendritic structure, but even when the experiment is carried out for several hours, no more than 1 to 2 kg snow can be produced. This means that this device is not suitable for producing snow for ski slopes.
European Patent Application Publication EP 609,140 A1 discloses the production of snow within a closed tunnel in which the snow circulates and which may therefore be used as a snow channel for testing various materials and articles under the influence of snow fall. Except for its moisture content, the characteristics of the thus obtained snow are not described, and the device described therein is not suited for the production of snow for ski slopes either.